This paper outlines Foucault's concept of governmentality and argues for its contemporary significance. It focuses upon the role that liberal modes of government accord to the exercise of authority over individual and collective conduct by expertise. The paper argues that nineteenth-century liberalism as a mode of rule produced a series of problems about the governability of individuals, families and markets and populations. Expertise provided a formula for resolving these problems instantiated in a range of complex and heterogenous 'machines' for the government of individual and collective conduct. Over the late nineteenth and early twentieth centuries one sees the rise of a new formula for the exercise of rule, which one can call 'the welfare state' - within which expertise becomes linked to the formal political apparatus in new ways. the strategies of rule generated under this formula of 'the welfare state' have changed fundamentally over the last fifty years. A new formula of rule is taking shape, one that we can perhaps best term 'advanced liberal'. The analytical machinery of most conventional political sociology - and most radical analyses that take their cue from Marxism - have not proved successful in characterizing these forms of rule nor evaluating their consequences. The forms of power that subject us, the systems of rule that administer us, the types of authority that master us, do not find their principle of coherence in a State, nor do they answer to a logic of oppression or domination. Analyses of governmentality can enable us to explore these relations between mentalities of rules, forms of truth telling, and procedures of expertise.